Web-FTP: cannot upload packed files

Hello,
I have tested ispconfig for two weeks now... very nice features.
Some questions... but step by step. First of all i think there is a
bug in Web-FTP. I am not able to upload packed files with extentions
*.zip, *.rar, *tar.gz etc. All I get is the message "FEHLER 103: Es wurde kein Username oder Passwort angegeben." and a new login prompt.
Creating directories or upload normal *.html files is no problem.